iio: adc: select buffer for at91-sama5d2_adc

commit 76974ef9 upstream.

We need to select the buffer code, otherwise we get build errors
with undefined functions on the trigger and buffer,
if we select just IIO and then AT91_SAMA5D2_ADC from menuconfig

This adds a Kconfig 'select' statement like other ADC
drivers have it already.

Fixes: 5e1a1da0 ("iio: adc: at91-sama5d2_adc: add hw trigger and buffer support")
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>